1. Field of the Invention
The present invention relates to a process for preparing a catalyst for the hydration of olefins into alcohols.
2. Description of the Prior Art
It is known that olefins can be transformed into alcohols in the gas phase under increased pressure with water vapor. Such processes have achieved a special technical importance in the production of ethyl alcohol from ethylene, and isopropyl alcohol from propylene. The syntheses of these alcohols is carried out in the presence of catalysts and, generally phosphoric acid applied to carrier serves as catalyst.
Carrier materials are known either on the basis of pure silicic acid (for example, siliceous earth or silica gel) or on the basis of silicic acid with a more or less high content of pure clay, such as, for example, calcined diatomaceous earth whose structure is held together by clay or clay-like materials.
In the case of carriers on the basis of pure silicic acid, the stability creates a problem over longer periods of time. Materials containing pure clay actually have a better mechanical stability but, with too high a content of pure clay, they have the disadvantage that the aluminum oxide is separated during the reaction, by the action of the phosphoric acid, leading to interfering deposits in the subsequently connected units.
It has also become possible to develop carriers for phosphoric acid with a high hydration activity and sufficient mechanical stability on the basis of coarse-pored silica gels, see for example, German Laid Open Patent Publications 26 25 705 and 27 19 055. However, there remains the disadvantage of these carriers on the basis of amorphous silicic acid that, with a longer exposure against the conditions of the hydration reaction, the amorphous silicic acid partially crystallizes to cristobalite and quartz, which is connected with a high reduction of the specific surface and thus of the catalytic activity in an irreversible manner, as well as with a decrease in the mechanical stability.
A process for the production of a pure clay-containing carrier has been described in German Patent No. 11 56 772 for the phosphoric acid used in the olefin hydration as catalyst in which molded contact bodies of mineral clay silicates, particularly commercially available bentonite-containing contact bodies for hydrogenations and dehydrogenations, are treated in such a way with mineral acid that the aluminum oxide content is preferably lowered to between 1 and 5% by weight. This material has the necessary mechanical stability as well as a sufficiently low remaining aluminum oxide content in order to avoid a deposit in subsequently connected units by components separated with phosphoric acid. In the course of the experience of many years with the use of commercially available bentonite-containing contact bodies for hydrogenations and dehydrogenations, it was now observed that, with the production of the commercially available contact bodies for the purpose of hydrogenation and dehydrogenation it is not necessary to the same degree to make a preliminary selection concerning the raw material as for the production of the carrier material for phosphoric acid. Thus, the hydration catalysts produced by acid treatment and impregnation with phosphoric acid from commercially available bentonite-containing contact bodies show greatly varying activities which leads to strong fluctuations of the plant capacity with existing reactor volume in continuously operating synthesis plants.
Since it has been determined in the meantime that the life of the catalyst is considerably prolonged by a continuous injection of the discharged amounts of phosphoric acid (German Laid Open Publication 26 58 946) and thus corresponding claims are also to be made to the life of the carrier, the use of such carriers is excluded in the case of which, under reaction conditions, a reduction occurs in the catalytic activity, for example, crystallizations take place in an irreversible manner and/or the mechanical stability is reduced in the course of time.